This invention relates to the treatment of substrates to prevent or at least greatly diminish the penetration into or through the substrate of liquid water while still permitting the passage into or through the substrate of water vapour.
The invention may be embodied as a method of treatment of the treated substrate itself, whether separate and capable of further fabrication into a final product or whether part of a permanent structure.
While the invention can be applied, for example, to the treatment of fabrics or the treatment of vapour-permeable layers such as natural or synthetic leather layers it can also therefore be applied to the treatment of a structural or decorative surface which needs to be permeable to water vapour while repelling or deterring the ingress of liquid water. In other words, the invention can be applied to the water-proofing of fabrics, to the surface treatment of leather, and to the surface treatment of walls, floors and ceilings to confer water repellency.
For ease of description the invention will be discussed primarily in terms of treatment of fabrics to confer water repellency.
It is well known to provide in apparel as a protective layer against rain, or against water droplets in the form of mist, an impermeable layer of polymer or the like. While suitable for short term wear, completely impermeable articles eventually generate considerable discomfort for the user, since the inevitable water vapour generated from the skin of the user cannot escape, accumulates at the inside of the impermeable layer and condenses. The garment becomes humid and uncomfortable to wear and becomes wet inside.
Because of this it is commonplace to treat fabrics to permit more or less passage of water vapour while preventing, or hindering, passage of water droplets. The water vapour molecule is many orders of magnitude smaller than a water droplet, and can pass through fine orifices which surface tension does not allow a water droplet to penetrate.
One known method of providing a "breathable" but water-repellent fabric is to utilize a fabric the threads of which expand upon contact with water. In normal use, the interstices between the weave are open enough to present no barrier to the passage of water vapour. If, however, the fabric is contacted with water the individual threads swell and tend to close off the gaps between them. There is still some facility for passing water vapour through such fabrics but the swollen moist fabric then prevents substantial transfer of water as liquid through from the outside to the inside.
There are, however, obvious disadvantages with such fabrics since they do become wet in use. Efficiency is not high, either for vapour transmission outwards or for the prevention of water travelling inwards.
A considerable improvement upon such materials can be obtained by conventional waterproofing, providing a water repellent coating for the fibres. This is the basis of the majority of waterproof materials, and involves treating the fabric in a solution of a water repellent polymer, typically a natural or synthetic rubber, so that a coating of such rubber is formed over each yarn or fibre. To some extent this has the effect of decreasing the size of the interstices between the fibres, but still leaving orifices through which water vapour can move inwards and outwards. However, the water repellent nature of the coating, defining the limits of these small interstices ensures that droplets of water do not penetrate the material (unless in extreme conditions). Water is therefore deflected while preserving the inside of the garment in a dry state. The differences between this and the foregoing proposal are that the threads themselves do not become wet and swollen, and that water is turned away from the fabric by the water-repellent coating on the threads.
A further improvement in fabric characteristics can be obtained by a process involving a different type of water-repellent polymer. If a film of polytetrafluoroethyethylene is produced e.g. by extrusion or casting, and thereafter stretched, it breaks up as an irregular network of fibrils while still retaining its essential nature as a sheet. Instead of stretching and tearing, it undergoes innumerable small fractures and cracks. It is known to take a sheet of such fibrillated or fractured film and to sandwich it between two layers of fabric, or possibly to adhere it only to a back of the fabric layer. Such a sandwich, or such a two-layer composition, is water repellent by virtue of the high water repellency of the PTFE. Water droplets, whether as rain or mist, are far too large to penetrate the small orifices between the fibrillated material, but of course these orifices, although small, present no barrier to the outward passage of water vapour molecules. Moreover, the fabric is free from any rubbery texture or smell, and the material has the advantage that it can be made up as a laminate from suitable reels of fabric and film, without the necessity for a dipping and curing process.
The present invention represents a new category of process for conferring water repellence upon a fabric while retaining vapour transmitting properties. It involves both the use of a water-repellent polymer, (such as a fluoropolymer) and the use of a dipping process, and gives a product where such polymer is present in a different physical state from anything envisaged or attainable by the prior art.
It is known in a totally different technical field to provide a non-stick coating upon for example a cooking utensil by sintering to the utensil PTFE powder previously applied as a suspension to the article and dried to a layer. It is also known, as an improvement or variant of this sintering process to provide a specialised curable paint vehicle containing particles of fluoropolymer with non-stick properties, and adhering this to a surface by curing the paint rather than by sintering the particles. In each instance, however, the objective is to obtain a good coating of material free from voids which can of course alter the non-stick properties and serve as sites for initiation of break up of the film. To this end, the surfaces to be treated are themselves typically quite smooth prior to application, so as to form a suitable basis for any provision of a non-stick sintered or cured layer.